Relaxation oscillator with variable frequency



Jan. 24, 1939. K. SCHLESINGER 2,144,779

RELAXATION OSCILLATOR WITH VARIABLE FREQUENCY Filed Jun e 11, 19:57

huenlfor Patented Jan. 24, 1939 UNITED STATES RELAXATION OSCILLATOR wrrnVARIABLE FREQUENCY Kurt Schlesinger, Berlin, Germany, assignor toRadioaktiengesellschaft D. S. Loewe, Berlin- Ste'glitz, GermanyApplicationJune 11, 1937, Serial No. 147,801

v p The need frequently arises, particularly in the case ofoscillographs, for a relaxation oscillation apparatus, the frequency ofwhich is able to be varied within wide limits, the relaxation poten- 5tial, however, remaining constant. If the oscillator is provided with agrid-controlled gas-filled thermionic tube this problem could be solvedsimply by a variation of the relaxation condenser. In practice, however,a variation in the capacity is comparatively expensive and complicated,particularly if a desired range of ill-10,000 periods is concerned,whilst resistance tuning would be cheap and practical.

It is an object of the invention to vary the frequency of a relaxationoscillation by varying its charging current and to provide a specialcompensation in order to maintain the potential of the oscillationsconstant. Further objects of the invention may be best understood byreference to the following description taken in connection with theaccompanying drawing, in which Fig. l is a connection scheme for varyinguniformly the relaxation frequency within moderate limits, whilst Fig. 2is a connection scheme for varying the frequency within wide limits.

In Fig. 1, I is the potential source, 2 the charging resistance, 3 therelaxation condenser and 4 the grid-controlled gas-filled thermionictube, which is connected with its anode and cathode to the condenser. Abraking resistance 6 is inserted in order to limit the dischargecurrent. As well known, oscillations of constant frequency which areindependent of the potential of the mains can only be obtained if thedischarge tube derives its grid bias from the charging current, so thatthe same is coupled with the current and accordingly with the mainspotential and participates in all variations in the opposite sense. Forthis purpose there is employed the resistance 5. If now the chargingresistance 2 were varied, the bias 5 would also vary. As a result therewould hardly occur the desired variation in frequency, but merely avariation in amplitude. This behaviour, although possibly to smallerextent, would always take place also in all other methods of generatingthe grid bias upon variation of the charging currents, which represent apart of the current loading the mains apparatus. Simple resistanceregulation of the relaxation frequency by varying the resistance 2 couldonly be performed, if resistance 5 would be replaced by a biassingbattery.

According to the invention, therefore, the following way is adopted: Thecharging resistance 2 Germany June 20, '1936 (01. ate-s0) is connectedwith a potentiometer l. The charging E. M. R, therefore, is varied atthe wiper thereof. An electric stop resistor 8 prevents thecharglhgpotential to drop below the ignition po tential-of the dischargetube. The potentiometer current flows also. through 5. Since the same issubstantially constant it furnishes the bias at 5 with a constant basiccomponent and already decreases the interfering dependency. According tothe invention, however, there is also caused by a resistance e anartificial increase of the charging current upon decrease of thepotential at l, as if the wiper of l is furnishing the full potentialthe resistance Q is short-circuited and ineffective. The more the wiperat l is moved downwards, the more is l shunted by 9, thus increasing thetotal current which reaches the grid resistance 5 along one of the twopaths, either by way of 2 or by way of 8. It is essential that thecharging resistance 2 is large as compared with the potentiometerresistance 1 and that the resistance 9 is approximately equal to theresistance 2. With the stated connection system it is possible, byshifting the wiper at l, to produce a variation in frequency such as 1:7with a practically constant amplitude. The igniting potential may beapplied to the grid in the usual way.

For additional amplification the oscillator is coupled with an amplifierE3 of any kind by means of a condenser I2 and a potential drivider I5,It. The significance of this potential distributor is as follows: Themore the same sets down the grid alternating potential, the greater arethe alternating potentials which require to be generated at the anode ofthe discharge tube, the greater, therefore, are the charging currentswhich require to be passed to the condenser 3, and the smaller is theeiiect of insulation faults and leakage or initial currents at thecondenser 3 or the tube 4 with respect to these charging currents to bemaintained constant. It is found that the ascent of the condenserpotential gains considerably in linearity if the charging currents areof the order of l m. a. instead of 0.1 m. a., provided a potentialsource i of a few hundred volts is used. This fact is caused byunavoidable insulation faults of the order of 10 ohms, correspondingalready to the initial current of such tubes.

Whilst by reason of the connection system according to the invention asshown in Fig. 1 it is possible to obtain a fine adjustment of thefrequency within ranges of approximately one order of magnitude, acoarse adjustment in powers of ten is obtained by switching over thecondensers. The connection system necessary for this purpose is shown inFig. 2, A multiple-way double-leverswitch I I is provided forchanging-over the condensers 3 together with its different brakingresistances 6 and the appertaining resistances 9.

I claim:

1. In a relaxation oscillator comprising direct current source, agas-filled grid-controlled thermionic tube shunted by a condenser inseries with a braking resistance, a grid biassing resistance between thecathode of said tube and the negative terminal of said source and acharging resistance between the anode of said tube and the positiveterminal of said source, means for varying the frequency, means to keepconstant the grid bias of said tube and consequently the dischargepotential of said condenser, said means consisting in a connection forkeeping constant the current flowing through said biassing resistancewhen the current charging the condenser is varied for frequencyvariation.

7 2. In a relaxation oscillator according to claim 1, the means to keepconstant the current flowing through said biassing resistance in aconnection guiding the whole current taken from said source through saidbiassing resistance and keeping constant said current.

3. In a relaxation oscillator according to claim 1, the means to keepconstant the current taken from said source consisting of variationmeans in the charging resistance.

4. In a relaxation oscillator according to claim 1, means to vary theoscillator frequency within wide limits in coarse steps, said meansconsisting of a set of charging condensers of difierent capacity each inseries with an adapted braking resistance, a set of difierent chargingresistances and a multiple-way double-lever switch inserting at once onemember of either of said sets.

5. In a relaxation oscillator according to claim 1, an electronicamplifier and means to couple the output of said oscillator with saidamplifier, said means consisting of a condenser in series to a potentialdivider, the intermediate terminal of said divider connected to the gridof said amplifier, said divider reducing the output potential of saidoscillator for at least one order of magnitude.

KURT SCHLESINGER. g5

